The investigation of mind is closely related to the field of epistemology, the part of philosophy that deals with knowledge and whose principal question is: “What can we know?” Epistemology is not so much preoccupied with the process of accumulating knowledge, but with the validity of knowledge and how we can achieve certainty about it. It includes the branch of philosophy that the ancients called logic, which deals with language and thought. Bertrand Russell once remarked tellingly that the theory of knowledge is a product of doubt. Things seem to speak in favour of Russell’s view – most philosophers find it easier to determine what we cannot know rather than what we can know. Perhaps the theory of knowledge should then be called “theory of ignorance.”The other question about knowledge is: “How do we know?” This question pertains to the mechanics of sensation, perception, cognition, memory, and physical brain processes. It also touches upon language and thought, but it takes a more scientific approach to these issues. The latter question is primarily asked by psychologists and neuroscientists, although philosophers recently took a renewed interest in the workings of the brain. Since both approaches are beneficial in their own way, we shall not limit ourselves to a particular one.